The present invention relates to an electrophotographic photosensitive body. In particular, the present invention is directed to an electrophotographic photosensitive body used in an electrophotographic device such as, for example, an electrophotographic printer or a copier. The present invention is directed to the improvement of a charge transport substance used for a photosensitive layer.
Conventional photosensitive materials for a electrophotographic photosensitive body (hereafter referred to as "photosensitive body") used for electrophotographic printers, facsimiles, digital copiers, and analog copiers include inorganic photoconductive substances such as selenium or its alloys; inorganic photoconductive substances such as zinc oxide or cadmium sulfide dispersed into resin binding agents; organic photo-conductive substances such as poly-N-vinylcarbazole or polyvinylanthracene, and organic photoconductive substances such as phthalocyanine or bisazo compounds that are dispersed into resin binding agents or vacuum deposited.
The photosensitive body requires properties that serve the functions of retaining surface charges in a dark place, receiving light to generate charges, and receiving light to transport charges. Further, photosensitive bodies can be classified into either single-layer photosensitive bodies having all these functions in a single layer or laminated photosensitive bodies having these functions separated into a layer mainly contributing to the generation of charges and another layer mainly contributing to the retention of surface charges in a dark place and the transfer of charges during the reception of light. Image formation using an electrophotographic method using such a photosensitive body is carried out by, for example, a Carson process. Image formation with this method is carried out by 1) charging a photosensitive body in a dark place using corona discharge, 2) forming on the charged surface of the photosensitive body, electrostatic latent images such as characters or pictures from manuscripts, 3) using toner to develop the electrostatic latent images formed, and 4) settling the developed toner images on a support such as paper. Once the toner images have been transferred, the photosensitive body is reused after static elimination, removal of remaining toner, and photo-static elimination.
Various image formation processes are used for electrophotographic apparatuses that use such a Carson process. The charging process includes a corotron or scorotron method that uses metallic wire and a contact charging method that uses a charging brush or roller. The developing process includes a two-component developing method, a non-magnetic one-component developing method, and a magnetic one-component developing method.
In recent years, electrophotographic photosensitive bodies of organic materials have been put into practical use due to their advantages such as flexibility, thermal stability, and film formation capability. Such photosensitive bodies include, for example, a photosensitive body of poly-N-vinylcarbazole and 2,4,7-trinitroflurenone-9-on (described in U.S. Pat. No. 3,484,237, incorporated herein by reference), a photosensitive body mainly comprising an organic pigment (JP-A-47-37543, incorporated herein by reference), and a photosensitive body mainly comprising an eutectic complex of a dye and a resin (JP-A-47-10785, incorporated herein by reference). Currently, common electrophotographic photosensitive bodies of such organic materials have a function-separated laminated structure in which 1) a charge-generation layer comprising non-metallic phthalocyanine, metallic phthalocyanine such as titanyl phthalocyanine, or an azo compound and a resin binder and 2) a charge--transport layer comprising a hydrozone, styryl, diamine, or butadiene derivative and a resin binder are laminated.
Photosensitive bodies can be charged negatively or positively. Generally, negatively charged photosensitive bodies are disadvantageous compared to positively charged photosensitive bodies. In a negatively charged laminated photosensitive body having a charge generation layer and a charge-transport layer formed on a conductive support (in that order) that also has holes moved due to the properties of the power-supplying charge-transport material so as to become sensitive when the surface is negatively charged, the corona discharge used in such negatively charged laminated photosensitive bodies during charging is unstable compared to positive charging. Furthermore, ozone or nitrogen oxides might be generated that stick to the surface of the photosensitive body, thereby causing physical and chemical degradation and affecting the environment. Thus, positive charging photosensitive bodies with fewer requirements have a wider range of applications and are more generally advantageous than negative charging photosensitive bodies.
As a result, in the prior art, various positive charging photosensitive bodies have been proposed. For example, a method for simultaneously dispersing a charge generation and transport materials into a resin binder to form a single photosensitive layer has been proposed and put into practical use to a limited extent. The sensitivity of this method, however, is insufficient for applications to fast devises and requires further improvement for repeatability. In order to provide a function-separated laminated structure to increase sensitivity, another method laminates a charge-generation layer on a charge-transport layer to form a photosensitive body for positive charging. Since, however, this method forms the charge-generation layer on the surface, the stability during repeated use is not good because of the effects from corona charge, light irradiation, and mechanical wear. In this case, a protective layer formed on the charge-generation layer has been proposed. Nonetheless, however, although this method prevents mechanical wear, it degrades important electrical characteristics such as sensitivity.
Other methods for laminating a charge-transport layer on a charge-generation layer to form a photosensitive body have been proposed using known charge transport materials that include 2,4,7-trinitro-9-fluorenone, but this substance is carcinogenic and thus unsuitable from the viewpoint of safety. In addition, although Japanese KOKAI's 50-131941, 6-59483, and 6-123986 have proposed cyano and quinone compounds, compounds having a sufficient charge-transport function for practical use have not been obtained yet.
As described above, organic materials have many advantages over inorganic materials, but materials having properties that sufficiently meet the requirements for the electrophotographic photosensitive body have not been obtained yet. There is a strong need for highly sensitive photosensitive body products that do not have their properties affected after long-time use in an electrophotographic apparatus. In particular, there is growing market demand for photosensitive bodies that can sufficiently endure long, continuous use in various electrophotographic apparatuses, including various image formation processes such as described above.
Conventional laminated organic photosensitive bodies have various problems to be solved, such as inadequate electrical characteristics such as photosensitivity, decreasing charging potential and sensitivity from extended use, and increasing residual potential from extended use under practical conditions. Thus, techniques that meet all the performance requirements have not been established.